This invention relates to a hydraulic brake-power booster unit for a vehicle having a dual-circuit brake system that is also equipped with an antilock system, and having separate power sources connected to separate proportional control valves for supplying pressure to each brake circuit. One of the proportional control valves is equipped with an emergency piston such that in the case of a power source failure, only the movement of the emergency piston, via the brake pedal, can be utilized to build up an output pressure for the corresponding brake circuit.
A hydraulic brake-power booster unit of this type is known from German Unexamined Published Application No. 30 15 689.
The known brake-power booster unit consists essentially of two proportional control valves arranged in a joint housing that are each assigned to a brake circuit of the motor vehicle brake system. These proportional control valves are developed as sliding valves that each have a valve piston or slider that is arranged in a prolate housing bore so that it can be slid in a pressure-sealed way. The valve piston or slider supports itself at an actuating member that can be slid back and forth by the actuating of the brake pedal by means of an elastic element acting as a path simulator, such as a rubber buffer or a helical pressure spring.
The piston of such a proportional control valve has an outer ring groove, one groove flank of which forms a control edge that, in the introductory phase of a braking process, travels more or less beyond an input duct to which the assigned hydraulic brake force source is connected. The distance travelled by the groove flank corresponds to the value of the pedal force. Due to this movement, an output pressure space of the proportional control valve itself is connected with the input duct via an axial bore of the valve piston and a radial bore which communicates with the ring groove of the valve piston. Thus, the respective brake circuit is dynamically acted upon, i.e., it is acted upon by a pressure that is derived from the output pressure of the outside power source.
By means of the pressure existing in the output pressure space of the respective proportional control valve, the valve piston is pushed back. In the state of equilibrium of the piston, i.e., when the output pressure corresponds to the expected value corresponding to the actuating force, this results in the closing again of the input duct.
In the normal position of such a proportional control valve corresponding to the unbraked state, the central duct of its piston communicates via another transverse duct with a balancing space that is connected with the brake-fluid storage tank of the brake system. The central duct is closed off on the actuating side by a piston flange of a valve piston. A self-supporting spring positioned against this piston flange forces the valve piston into its normal position. Path-simulating spring elements are arranged between these piston flanges and the actuating member of the brake-power booster unit that can be shifted by the actuating of a pedal.
In the case of the known brake-power booster unit a third housing bore is arranged in parallel between the parallel housing bores receiving the valve pistons of the proportional control valves. A so-called emergency piston, that is sealed off with respect to this housing bore, is slidably arranged in this third housing bore. This emergency piston is arranged between an input pressure space, that is connected with the output pressure space of one of the two proportional control valves, and an output pressure space, that is delimited by a section of the central housing bore. The output pressure space is connected to one of the two brake circuits, such as the front-axle brake circuit, of the vehicle.
The emergency piston has a central longitudinal through-bore, the mouth edge of which on the side of the input pressure space forms the valve seat for a ball valve. The valve body of the emergency piston is forced into the open position of this valve by means of a helical pressure spring supporting itself at the emergency piston. In addition, a piston flange, that on one side delimits the input pressure space, is guided in a pressure-sealed manner in the central bore. The piston flange is rigidly connected with the actuating member of the brakepower boosting unit and is therefore coupled with the brake pedal with respect to movement.
When the outside power source is intact, the two brake circuits connected to the known brake-power booster unit are dynamically acted upon via the proportional control valves. The ball valve of the emergency piston, that carries out the movements of the actuating member of the brake-power booster unit remains open because the emergency piston on both sides, is acted upon by the output pressure of one of the proportional control valves.
In the case of a failure of the hydraulic outside power source, (at least for that brake circuit having a proportional control valve connected to the input pressure space delimited on one side by the emergency piston), and upon an actuating of the brake system, the ball valve of the emergency piston changes into its closed position. As a result, the output pressure space that is delimited by the emergency piston is closed off with respect to the input pressure space.
In this situation, the emergency piston acts like a "normal" master brake cylinder piston, the shifting of which causes brake pressure to build up in the output pressure space. The emergency piston is provided so that the pressure accumulators in the outside pressure source can be kept as small as possible while ensuring that even after a breakdown of the pump, when accumulators become exhausted, braking can still take place with the legally required minimum vehicle deceleration.
However, it is a disadvantage in the case of the above known brake-power booster unit that the tightness of the ball valve required for the operability of the emergency piston during the driving operation of the vehicle can no longer be practically tested because it exercises its closing function only in the case of a breakdown of the outside power source. Corrosion and dirt related leaks of this ball valve, the statistical frequency of which under raised safety conditions cannot be neglected, as such conditions cannot be recognized by the system. Therefore, a situation of considerable potential danger results, especially in the case of a failure of the accumulator charge pumps when the accumulating capacities are relatively small.
It is therefore an objective of the invention to improve a brake-power booster unit of the above-mentioned type to the extent that a faulty situation resulting in the ineffectiveness of the emergency brake system comprising the emergency piston can be reliably recognized and prevented.
According to the invention, this and other objectives are achieved by a brake-power booster unit dynamically acts upon only one brake circuit, whereas the other brake circuit is braked statically. In the case of a vehicle having a front-axle/rear-axle brake-circuit division, the rear-axle brake circuit is the dynamically acted upon circuit, i.e., it can be acted upon by the output pressure of a proportional control valve that can be actuated via a path simulator. The front-axle brake circuit is braked statically, in which case the brake pressure for the front-axle brake circuit is built up in an output pressure space that is delimited on one side by the emergency piston of the brakepower booster unit.
The proportional control valve for this statically acted upon brake circuit is integrated into the emergency piston and generates an output pressure that acts on its working surface. The piston of the proportional control valve, developed as a slider, is an extension of an auxiliary piston that is movably coupled with the brake pedal.
In the case of a breakdown of the pumps of the outside power source, the auxiliary piston supports itself directly at the emergency piston. In this way, brake pressure can be built up in the output pressure space assigned to the front-axle brake circuit by pedal force.
The shifting path of the emergency piston is monitored by a path transducer. The pressure at the output of the output pressure space of the brake-power booster unit, that is delimited by the emergency piston, is monitored by a pressure transducer. These output signals of the path transducer and of the pressure transducer are fed to an electronic comparison unit. If this pressure does not correspond to the desired value associated with the respective position of the emergency piston, the electronic comparison unit generates a warning signal. By means of the warning signal, it is recognized that the shifting path of the emergency piston is too large compared with the acquired value of the output pressure. By means of this monitoring, all statistically important cases of malfunctioning or danger situations can be recognized in time and preventative maintenance measures can be initiated.
It is also an advantage that no valve is required that is susceptible to failure corresponding to the ball valve of the known brake-power booster unit. A further advantange is that because of the integration of one of the two proportional control valves into the emergency piston, the brakepower booster unit can also be constructed with advantageously small radial dimensions resulting in favorable installation conditions.
In the case of the preferred development of the brakepower booster unit according to the invention including a reversing valve which permits flow from the supply unit to an assigned brake circuit, the brake circuit that is normally statically acted upon by pressure, starting from a certain position of the emergency piston, is dynamically acted upon by pressure. As a result, in the case of a long-lasting braking that may lead to vapor lock, more brake fluid can be fed in. Thus, the emergency piston practically remains stationery. Also, it is ensured that after a pressure reduction phase of the antilock control, the brake circuit cannot be "controlled to be empty", and that for a possibly required subsequent emergency braking, a sufficient shifting path is still available for the emergency piston.
The switching of the brake circuit, from a static to a dynamic operation by means of the emergency piston, may take place by coupling the output pressure of the assigned pressure accumulator to the output pressure space assigned to this brake circuit.
The development of the brake-power booster unit according to the invention includes a reversing valve connecting the power source to the output pressure space via an anti-cavitation space.
The switching of the brake circuit that can be acted upon by pressure by the shifting of the emergency piston from this static to the dynamic operation can be controlled electrically, i.e. by means of an electromagnetic valve that is controlled by means of the path transducer. However, it is more advantageous if, as provided according to claim 4, a mechanically actuable reversing valve is provided. In which case, the piston stroke of this valve is monitored by means of the path transducer. This construction results in a simpler and more cost-effective construction and facilitates a constructional integration of the reversing valve into the brake-power booster unit.